1. Field of the Invention
The present invention generally relates to a medical diagnosis system and, more particularly, to a skin diagnosis system and method employing multispectral imaging.
2. Description of the Prior Art
The use of infrared photography as a potential tool for diagnosing tissue has been known for many years. More recently the use of photography to diagnose degrees of skin conditions, e.g., degrees of burns, has been proposed. The proposed diagnosis of burns is based on the use of visible and near-infrared photography. This proposed diagnosis is based on the fact that: (a) various wavelengths of light penetrate different depths into the skin; and (b) various types of tissue exhibit different degrees of light absorption at different wavelengths.
The recently proposed diagnosis of burns is based on the production of red, green, and infrared spectrally-filtered photographic images of the burned skin of a patient for determining areas of full-thickness burn as well as for differentiating between areas of full and partial-thickness burns. In accordance with the proposed burn diagnosis system three pictures are taken simultaneously through infrared, green, and red filters with three adjacently-located cameras of the area of interest.
After proper photographic processing three different photographs (transparencies) are produced. The first one represents the difference of the optical densities of the pictures, taken with the infrared and red filters. The second transparency represents the difference of the optical densities between the photographs, taken with the infrared and green filters, and the third transparency is one representing the difference of the optical densities of the pictures, taken with the red and green filters.
From these studies it has been found that one can determine the depth of the burn, i.e., whether it is a full-thickness or partial-thickness burn by observing the relative values of these differences of optical densities over a post-burn period. Also these three negatives are used to produce a color picture, which is also used in the diagnosis of the depth of the burn.
It is of great importance to diagnose the depth of the burn as early as possible in order to determine how the patient should be treated. Early primary excision of the full-thickness burn reduces the risk of infection, fibrosis, and loss of function. It also appears to provide the best surgical results in the shortest time. In many cases, however, it is very difficult to differentiate clinically between full and partial-thickness burns. In these cases the diagnosis depends heavily on the intuition and experience of the physician. Although the physician is aided by many tests covering sensory, mechanical, and thermal phenomena, the only positive method available to him is to wait three to four weeks until the natural healing pattern is established and the areas of irreversible damage are indicated. During the first weeks after the injury the wound is characterized by three zones: the zone of hyperaemia, the zone of stasis, and the zone of coagulation.
The zone of hyperaemia represents the most superficial wound. This zone appears red and continues to exhibit adequate circulation and metabolism necessary for viability. By the seventh day the wound is dry and healed with complete regeneration of the epidermis, but no apparent damage to the dermis.
The zone of stasis appears very much like the zone of hyperaemia, but upon testing, one finds the absence of metabolic processes. At the end of 24 hours, circulation ceases and complete stasis occurs. Between the third and seventh days, the zone of stasis turns brownish-white because the superficial surface of the dermis is avascular and necrotic, and the red cells which have previously colored the zone have been hemolyzed.
The zone of coagulation appears brownish-white and is characterized by complete obliteration of the lumina of the vessels and the subpapillary plexus, in the coagulation of the tissue.
It is desirable to excise the full-thickness burn early, during the first week if possible. Although it is known that the tissue of the zones of coagulation and stasis will be lost, it is not possible by inspection to determine whether these zones will include the full thickness of the skin or not. If excision is performed before a precise diagnosis of the full-thickness burn is made, the patient may suffer because sufficient tissue was not removed. Residual necrotic tissue will then remain as a source of sepsis requiring additional debridement, accompanied by additional pain and expense. If an excess of tissue is removed initially, the patient may suffer because he has lost viable tissue necessary for the healing process. Any delay or inaccuracy in the diagnosis of the full-thickness wound therefore increases the danger of sepsis. Sepsis is the single most common cause of death for the burned patient.
The prior proposed method of diagnosing burns of producing actual photographs on film which are then processed, as herebefore described, is capable of serving as a diagnostic aid. However, it has several very significant disadvantages. Ignoring cost, its primary disadvantage is the relatively long time period which has to elapse from the time the original pictures are taken until the final color pictures are produced. It is estimated that the time period would be about four to six weeks, which in most instances would be longer than absolutely required for proper treatment. This relatively long time period is due to several factors.
First, the optical density produced on any photographic film is related to various factors including the logarithm of the reflectance of light from an object to the film. Thus, to produce the ratio of the reflectance of light of different wavelengths from a subject which is directed to different cameras, the optical densities of the exposed films in these cameras has to be subtracted. Since in the proposed method three photographs are taken with three different cameras, even though they are located very close to one another, the resulting photographs have to be registered in a nonlinear fashion to insure the corresponding incremental areas (pixels) in pairs of these photographs overlay one another to produce correct optical ratio outputs. Also, picture enhancement is often required to increase the range of contrast. This is achievable by varying the .gamma. of the film. Only then can the ratios of the reflectance to corresponding pixels of different pairs of films be achieved, by subtracting the optical densities of the corresponding pixels on these films. Only thereafter can three transparencies be produced to finally produce the final composite color photograph which is used for the actual diagnosis.
It is estimated that even if one had full access to a relatively large and specialized data processing system, it would take at least a week to properly process the original three filtered photographs and produce the final color photograph for the burn diagnosis. Clearly, such lengthy processing, requiring highly experienced personnel would also be very costly. Thus even though the prior proposed burn diagnosis by means of taking different filtered photographs of the area of interest is feasible, it is not practical, nor is it capable of providing the diagnostic information early enough to help in the proper burn treatment. A need therefore exists for a new system and method for diagnosing skin conditions. It should be appreciated that a person's skin is but one type of surface tissue. Thus, generically a need exists for a new method and system for deriving information related to surface tissue conditions.